It is well-known that very high contrast photographic images can be formed using certain silver halides, and methods for forming such photographic images have been employed in the photomechanical process.
One of such known methods involves in using a lith type silver halide photosensitive material which comprises fine-grained silver chlorobromide (of which at least 50 mol% or more is silver chloride) having mean grain size of 0.5 micron or less and a narrow grain size distribution. By processing a material of this type with a hydroquinone-containing developing solution in which the effective concentration of sulfite ion is extremely lowered (generally 0.1 mol/liter or less), line or dot images having clear distinction between the image areas and the nonimage areas (that is, high contrast) and high density in blackened areas can be produced. In this method, however, the developing solution used is quite unstable to air oxidation because of its low sulfite ion concentration. Therefore, in the present situation, it is used with many efforts and various devices for keeping the solution activity stable.
Such being the case, image-forming systems which can solve the instability of image formation using the above-described developing method (lithographic developing system) and can provide super high contrast photographic characteristics in spite of the development with a processing solution having excellent storage stability have been desired, and which the intention of satisfying this desire, U.S. Pat. Nos. 4,166,742, 4,168,977, 4,221,857, 4,224,401, 4,243,739, 4,272,606 and 4,311,781 have proposed a novel system that a surface latent image type silver halide photographic material in which a particular acylhydrazine compound is incorporated is processed with a developing solution excellent in storage stability, which has a pH value of from 11.0 to 12.3 and contains 0.15 mol/liter or more of a preservative of the sulfuric acid type. This enables a super high contrast negative image having a gamma value of 10 or above. This image-forming system has the merit of making it feasible to form high contrast images using silver iodobromides or silver chloroiodobromides, as well as silver chlorobromides having high silver chloride contents, as compared with conventional systems for forming high contrast images in which only silver chlorobromides of high silver chloride contents can be used.
However, it sometimes happens that this novel image-forming system causes undesirable phenomenon of black spots due to infectious development occurring simultaneously with marked increases in photographic speed and contrast. The generation of such black spots is a problem remaining to be solved in the photomechanical processes.
The term black spots refers to fine specks of developed silver which appear in the unexposed areas to be normally non-image areas. The black spots tend to appear in great numbers, particularly when exhaustion of the processing solution with the lapse of time causes the rise in pH or so on. Under these circumstances, considerable efforts have been attempted to prevent the generation of the black spots, but improvements with respect to the occurrence of black spots have frequently been accompanied by decreases in photographic speed and lowering of image contrast. Therefore, such a system as to suppress the generation of black spots as it attains highly sensitive and super high contrast photographic characteristics has been awaited.
On the other hand, a silver halide photographic material, the photographic density attained by a per-unit-area amount of developed silver becomes, in general, higher with smaller size silver halide grains, whereas the sensitivity of silver halide becomes, in general, higher with greater size silver halide grains. Therefore, it is necessary to employ a silver halide emulsion having a large grain size in a larger amount per unit area if one wishes to obtain a photosensitive material having high sensitivity and high photographic density. A photosensitive material containing a large amount of silver halide emulsion, however, requires much time at the stage of development, and further for effecting fixation, washing and drying, to result in a loss of rapid processability. In addition, silver is an expensive resource, and its production and reserves are limited in quantities. Consequently, it is requested to produce a photosensitive material using the smallest possible amount of silver.
From these viewpoints, investigations have been undertaken for many years to produce silver halide photosensitive materials of both high image density and high photographic speed using a reduced amount of silver. As a result of such investigations, Japanese patent application (OPI) 58137/82 (The term "OPI" indicates an unexamined published patent application open to public insepection) discloses the method that the mixture of a high photographic speed emulsion having grain sizes of 0.7 microns or more with an emulsion having grain sizes of 0.4 microns or less is processed in the presence of a hydrazine compound to acquire the photographic characteristics of high photographic speed and high contrast. However, this method also cannot attain such high contrast (gamma of not less than 10) and high photographic density as is some times needed in the photomechanical process, and further suffers from marked generation of black spots, as cited for comparison in the Example.